Hard reset done but still stuck

Hi
I am trying to help out my eldery neighbours. Their tablet consistently asks for the previuosly synched Google account email info even after a hard reset. They did not have one when it was set up by their grandaughters, just a Yahoo email address and a phone number. Google will send a verification code to the phone number which works but when putting in the paswword an error message pops up saying “ Unable to proceed due to unknown error. Try with another email address or again in 24 hours”.

How can I get around this! Thx.